"The least of Reptiles I have hitherto met with, is a Mite, a Creature whereof there are some so very small, that the sharpest sight, unassisted with Glasses, is not able to discern them... Notwithstanding which minuteness a good Microscope discovers those small movable specks to be very prettily shaped Insects ..."
So wrote Robert Hooke in 1665 in his widely acclaimed work `Micrographia' dedicated to King Charles II amidst the last great plague of London and a year before the great fire which destroyed 13,000 buildings but surprisingly claimed the lives of only four people. Certainly fraught times for a Londoner to live through, whilst concerning himself with dust mites.
The house dust mite is neither a reptile nor insect but until Carolus Linnaeus brought some assemblage of order into classification in 1757 with his Systema Naturae, a scheme of binomial nomenclature, little serious attention was paid to such detail. Employing his basic microscope with reletively poor lenses, Hooke described the mites in considerable detail with remarkable accuracy carefully stressing the ".. eight well shaped and proportioned legs ..." if comparing them still further with crabs and lobsters. Had he lived to see the 300th anniversary of his work, he would have witnessed the marketing of the first commercially available scanning electron microscope, by Cambridge Instruments, which surely would have astounded him.
Modern classification places mites with the spiders and though their classification appears to be in a state of flux, house dust mites may be classified as follows:
Phylum - Arthropoda
Class - Arachnida
Order - Acarina
Sub-Order - Sarcoptiformes
Division - Astigmata
Family - Epidermoptidae
Genus - Dermatophagoides
Species - D. farinae, D. pteronyssinus
It is alleged that house dust mites inhabit our homes by the millions; our beds, furniture and carpets are over-run with them. Manufacturers of vacuum cleaners and producers of mite-proof, `anti-allergenic' fabrics for bedding materials market their products by trading excessively on this general perception. Adult dust mites are very small, approximately 300um in length, so head to tail three or four approximately span a millimetre and are almost invisible to the unaided eye under normal conditions. Males tend to be smaller and more flattened than the female which takes on a rotund appearance especially when fully mature and carrying eggs.
The tough leathery skin is mostly translucent allowing the internal organs and haemolymph to impart an overall creamy white appearance to the body with isolated patches of pale yellow. Sclerotised areas, such as the legs and head of fully mature adults, are more heavily pigmented with a red/brown colouration that relieves the otherwise bland exterior. Microscopically the most striking and aesthetically pleasing feature of the skin is the sculpturing that resembles whorls of a finger-print.
The house dust mite is devoid of a true head, this has evolved into a complex mouthparts structure known as the gnathosoma with which it feeds on hair debris, flakes of skin and general detritus.
Eyes are also absent but on each side of the body immediately above the first pair of legs there exists a bristle, the supra-coxal setum, associated with an elongated pore located in a recess . The bristle's architecture varies considerably between the genera of mites (being the least spectacular in Dermatophagoides spp.) and for the acarologist is useful for identification purposes but undoubtedly is a highly developed sensory organ essential for the mites survival. Though there are no obvious light receptors house dust mites are extremely photophobic and become very animated to seek dark recesses when brought into the light.
The life-cycle of the house dust mite is a three stage process; egg, larva (nymph) and adult. As there is no distinct pupal stage, metamorphosis is said to be incomplete. The larval stage however is protracted and as the nymph develops and grows, it sheds its skin three times before becoming a fully formed, sexually mature adult. From a visual perspective nymphs closely resemble a smaller version of the adult form with two notable exceptions; they are mostly devoid of colour and more obviously have only three pairs of legs. Though visible as `buds' during the third nymphal instar, the hind (fourth) pair of legs is not present until the adult mite emerges from the final ecdysis. The entire cycle from egg to adult spans 2 to 3 weeks, with the adult living for about a month, but is very variable depending upon the ambient temperature and relative humidity of the environment.
House dust mites can tolerate a wide temperature range from around 0 degrees C (in a moribund condition) to a maximum temperature of about 30 degrees C with 20 - 25 degrees C being the optimum for breeding. Humidity of the environment is more important and there is less tolerance to variation where the optimum relative humidity is about 75% with a maximum deviation of +/- 10%. Whilst most centrally heated homes probably maintain a constant ideal temperature the humidity generally will be lower than 65% perhaps not making these types of buildings as attractive to the mite as one is led to believe.
Sexually mature adults mate by the male initially climbing onto the back of the female with both individuals facing the same direction but the male quickly reorientates itself through 180 degrees to take up a retroconjugate mating position. In nearly all cases of matings observed by the author this activity resulted in the female becoming very quiet and totally immobile as though influenced by a male initiated pheromone.
The male maintains position on the back of the female by two large paranal suckers on the underside of the abdomen.
The copulatory organ of the male is located approximately half way down the body on the ventral surface and protected by two paragynal lips (upper image, left) through which the penis protrudes by haemolymphatic pressure during copulation enabling penetration of the genital/anal pore (upper image, right) of the female.Once insemination has occurred, the male abandons the female which, from direct observations, appears to take tens of minutes to recover full mobility.
The female subsequently lays white, relatively large, eggs singly at random by extrusion from the ovipositor, located centrally in the ventral surface, which hatch after a short incubation period of two or three days depending upon environmental conditions.
House dust mites have been identified as a source of allergens for those individuals sensitive or allergic to house dust components. It has now been established that rather than the mite itself as the causative agent of the reaction, it is the faecal matter the mite excretes and more specifically a residual enzyme it contains that is responsible for most allergies. Faecal pellets are approximately 10um to 20um in diameter, the same size or smaller than many pollen grains, dry and susceptible to becoming statically charged. If the pellets are accidentally disturbed into the air they can remain airborne for long periods of times and in this respect have similar properties to pollen, all too familiar allergens of hay fever.